The Bow-Tie Brigade

Many MiniCoupe builders successfully use the tried and true VW derived conversion for power.  That is the engine that Johnson envisioned; however, the design is robust enough that the smaller A65 and C85 Continentals have also been used with satisfactory results and Ron relates that up to an O200 is acceptable.  Needless to say these last are somewhat heavier engines, and in my case we’re going to trust the airframe with even more weight – that of a Chevrolet Corvair.  I find absolutely nothing inappropriate with the VW derived engines, it is just that I have a Corvair originally intended for a different airframe – so I’ll be adapting it for both budget and time reasons.

By now the Corvair, although nowhere near as accepted as the Volkswagen derived power plants, is not a mystery in homebuilders’ circles – with many KRs and Zenith CH600 series aircraft using this larger engine to good advantage.  The Achilles heel of the Corvair seems to be that at higher output, the crankshaft can become susceptible to vibrations and gyroscopic loadings that Chevrolet never envisioned, making the nitrated crankshaft a must for this conversion -- and increasingly the 5^th bearing (prop-hub) has become an implicit requirement for any of the higher-performance installations.

At least at the outset I’ll probably be using the trusty Keihin CV40 carburetor often used on Harley Davidson and Kawasaki motorcycles, among others.  The CV40 is a moderately altitude compensating carburetor – essentially akin to the CV Bings used by Rotax. – and again, it is what I have and am familiar with, so hope no additional investment will be required.  Further, I will be using the rare TRW forged pistons – in my case we purchased the 0.030 over-bored cylinders and pistons already painted and stored, awaiting eventual reassembly with the engine.

For the MutantCruiser, however, we will not be designing in as much Chevrolet power as is usually may be available from the Corvair.  Typical, for instance, the big 3100cc KR Corvair engines can churn out up to 135-140hp which is far in excess of what the MiniCoupe airframe can adequately use.  As I had planned in two early building attempts, my Corvair will be tuned closer to the level that Bernie Pietenpol used to good advantage in his admired AirCampers – about 85hp-88hp.  Although that is fairly small by Corvair standards, it is a nod to longevity and it is further envisioned that cruising output will be throttled back in the neighborhood of 55hp at a little less that 2700rpm, for a roughly 5-gallons an hour fuel burn. 

With reasonable consideration to aerodynamic cleanliness and a well-tuned engine, I am hopeful for a cruise in the neighborhood of 105-108mph which should offer sensible cruise fuel economy along with respectable power reserves for climb-out on hot summer afternoons -- of course faster would be better, but not at the expense of greater fuel burn.

We shall see…

MutantCruiser -- we begin again...

My design goal is for a homebuilt single-seater that fully meets the spirit of the FAA light-sport aircraft and pilot regulations -- an aircraft that remains uncomplicated to fly for a lower time pilot with a two decade hiatus and an aircraft that is easy and inexpensive to build, ultimately providing a modest cross-country ability for chasing after grandma and the grandkids, and yet is modestly capable of skirting complicated airspace which unfortunately is a fact of life where I live.

Many devotees will correctly recognize my proposed aircraft as a MiniCoupe.  The MiniCoupe is one of the oldest of the species of diminutive aluminum single-seaters such as the BK1 and the Thatcher CX4 as well as the venerable Hummel series of aircraft refined by Mory Hummel and Bill Spring.  The MiniCoupe, also know as the Chris Tena MiniCoupe, (except for its twin-tails) is visually reminiscent of the Teenie Two -- although a noticeably larger and heavier aircraft compared to the Teenie Two, to the extent that any aircraft in this size range can really be called "heavy." 

For whatever reason, far more Teenie Twos are recorded than MiniCoupes, but there remains a relatively active discussion group for both aircraft, thanks in no small part to the efforts of Ron Dixon who has kept both designs very much alive through his plans sales and vigorous encouragement.  Ron was an early builder of the MiniCoupe and decades later still maintains his in Maryland.

Designed by Bill Johnson of Oregon in 1970s, the MiniCoupe was intended to provide an amateur builder with a single-seat aircraft,  via an easy-to-build and complete materials kit where most of the hard technical work was already finished – a concept now common-place in the experimental aircraft industry under the 51% rule.  Although all contemporary MiniCoupes are now pure plans-built, Ron Dixon notes that the original MiniCoupes were the first of the modern materials kits, and at the time the notion of a "kit" was an entirely new and revolutionary home-builder’s concept.

The MiniCoupe is essentially all aluminum construction and generally uses pulled/blind rivets  with 2024T3 aluminum throughout, although some have asked about using conventional riveting, especially in high-stress areas and others have actually used flush-riveting in hopes of lowering drag and expanding the flight envelope upwards – reputedly with good success.  Like most others of this genre, the Tena MiniCoupe was primarily intended as a VW powered airplane, although other engines have been used very successfully and a few use the smaller 65hp and 85hp Continental – mine will use a Chevrolet Corvair conversion (more on this another time).

For a variety of reasons, my MiniCoupe will be whimsically known for now as a the MutantCruiser, rather than trying to convince anyone that my work is a genuine MiniCoupe – but aficionados will quickly recognize the robust MiniCoupe structure, as well as general composition and minimalist philosophy that is plagiarized throughout.  The key reason for the nomenclature tweaking is that I may end up taking a few liberties with the Johnson design and I have zero interest in debating whether my altered rendition is properly called a MiniCoupe, nor am I interested in drifting into some of the testy parochial debates that rage through aviation forums on the internet when a builder decides to stray from the established blueprints of a respected designer.  I suspect if we all wanted pure clones, we'd fly Cessnas.

In any case, because of my heavier Corvair engine and the portly geriatric pilot destined to occupy the cockpit, the poor MiniCoupe would probably be near, or at, its designed gross weight all of the time, before we added any gas – and I have no intentions of building a glider!  Nonetheless, thanks to modern calculators and readily available EAA spreadsheets, I have run dozens of possible scenarios – keeping a wary eye on the fact that I am neither an engineer nor an A&P, nor even an experienced builder -- just a wayward hobbyist.  Further, although I’ve made hesitant starts on two different aircraft in the past two years, I’m now thoroughly chastened that although it is incredibly easy to modify a proven design in the virtual-world with the aid of modern computing, unless one is extremely carful, the real-world construction complexity goes through the roof at about the same rate that the probable fly-ability plummets. 

My target is for a simple yet modestly efficient single-seater that falls under the FAA light-sport aircraft and pilot regulations; a little bird that remains comfy to fly on up to 500-miles VFR XC hops and that is quite easy and absolutely inexpensive to build.  Further it must use as much of the materials and assemblies as possible from my two abortive building efforts.  Proffesional planners will recognize this as your basic “time, scope and resources” conundrum. Because social-security is quickly approaching, we have limited time and very restricted resources, so although my dream RV-3 would be nice and a fantasy Lancair IV superb, my real-world scope must remain modest – even austere!

I’m just a geriatric first-time builder who hears the clock ticking -- and there ain’t no free lunch!

Links:
http://au.groups.yahoo.com/group/minicoupe/?yguid=244371447
http://theminicoupe.com/id2.html
http://wday6511.home.comcast.net/~wday6511/